Recently, as the recording density for information recording mediums is being improved, the shortest mark length of recording marks is approaching the limit of the resolving power, which relies on a detection system.
In the case where, for example, an information recording medium is an optical disc medium, the “resolving power, which relies on a detection system” means an optical resolving power, which relies on the size of an optical spot generated by collecting laser light.
Due to the limit of the resolving power, increase of the inter-code interference and deterioration of SNR (Signal to Noise Ratio) are more conspicuous. As a result, a PRML (Partial Response Maximum Likelihood) method or the like is now used generally as a signal processing method.
The PRML method is a technology generated by combining partial response (PR) and maximum likelihood (ML). By the PRML method, with a premise that known inter-code interference occurs, a signal sequence having the maximum likelihood is selected from a reproduction waveform.
Owing to this, the PRML method is known to improve the decoding capability than a conventional level determination method (e.g., Non-Patent Document No. 1).
In the meantime, because the signal processing method is now changed from the level determination method to the PRML method, problems occur in the evaluation method of a reproduction signal.
Jitter, which is a conventionally used reproduction signal evaluation index, is used with a premise that signal processing is performed by the level determination method. Therefore, occasionally, jitter may not be correlated with the decoding capability of the PRML method, which is based on a different signal processing algorithm from the level determination method.
Under the circumstances, new indices which are correlated with the decoding capability of the PRML method has been proposed (e.g., Patent Document No. 1 and Patent Document No. 2).
Now, a case where the recording and reproduction quality is detected as distributions shown in FIG. 25 will be discussed.
FIG. 25 shows four distributions of differential metrics classified by the length of the space of 2T, 3T, 4T or 5T combined with a 3T-long mark, and the total of the four distributions. T represents a channel clock.
In FIG. 25, only 3T-long marks are classified as an example, but usually marks of other lengths are also classified.
In the case where classification is made by the length of the mark and the length of the space, there are the following two cases. In the case of FIG. 25(a), an SN component, which is the distribution width, is dominant in the distributions of in all the mark-space combinations. In the case of FIG. 25(b), an SN component of each pattern is good, but a shift component from the center of the distribution is different among the patterns. When the distributions are summed up, it appears that the SN component is poor.
The index described in Patent Document No. 1 cannot distinguish whether each distribution of differential metric is caused by an SN component or by a shift component.
Patent Document No. 3 solves this problem.
The index proposed in Patent Document No. 3 can detect a positional shift between a mark and a space (edge shift) by a combination of a mark length and a space length.
Namely, the level of the recording and reproduction quality obtained by the index proposed in Patent Document No. 1 can be distinguished as corresponding to an SN component or as corresponding to a shift component.
Owing to such distinguishing between an SN component and a shift component, it is now possible to analyze which type of error occurs in which pattern.
As described above, as the recording density of information recording mediums is more improved, the problems of the inter-code interference and SN deterioration will be more serious.
It is described in Non-Patent Document No. 1 that the system margin of an information recording and reproduction apparatus can be maintained by using a higher-order PRML method.
For example, when the recording capacity of one recording layer of a 12-cm optical disc medium is 25 GB, the system margin can be maintained by adopting the PR1221ML method.
The above-mentioned book describes that when the recording capacity of one recording layer is 33.3 GB, the PR12221ML method needs to be adopted.